Recently, there has been an increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Particularly, according to the recent trend that electronics become downsized and lightweight, the rechargeable secondary batteries also need a smaller size and a light weight, as well as high capacity.
Also, the secondary batteries are classified according to the structure of an electrode assembly consisting of cathode/separator/anode, for example, into a jelly-roll (winding type) structure obtained by interposing a separator between a cathode and an anode which are in the form of a long sheet, followed by winding, or a stack (laminating type) structure obtained by interposing separators between multiple cathodes and anodes having a predetermined size, followed by sequentially laminating.
However, such a conventional electrode assembly has several problems.
First, since the jelly-roll type electrode assembly is prepared in by winding a cathode and an anode being a sheet form in the contact state thereof to form a cylindrical or oval cross-section, the electrode assembly is internally accumulated with stress caused by the expansion and contraction of electrodes during charging and discharging processes. When such a stress accumulation exceeds a certain limit, the electrode assembly is apt to be deformed. By the deformation of the electrode assembly, the space between the electrodes become un-uniform to deteriorate battery performances rapidly, and an internal short circuit is generated to threaten battery safety. Also, the winding of the cathode and the anode being a sheet form is difficult to maintain the uniform distance between the cathode and the anode, and the rapid winding thereof is also difficult, and thus it is unfavorable in terms of productivity.
Second, since the stack type electrode assembly is prepared by laminating multiple cathode and anode units sequentially, it needs a separate procedure of transferring a plate for producing the units, and the sequential laminating requires much time and efforts, and thus its productivity is low.
In order to overcome these problems, the present applicant has developed a distinctive-structured electrode assemblies being a mixed form of the jelly-roll type and the stack type, i.e., stack-folded electrode assemblies prepared by winding bi-cells or full-cells with a long separator sheet continuously, the bi-cells or full-cells being obtained by laminating cathode units and anode units between which separators are interposed. Such stack-folded electrode assemblies are disclosed in Korean Patent Application Publication NOS. 2001-0082058, 2001-0082059 and 2001-0082060.
FIGS. 1 to 3 are cross-sectional views schematically showing the structure of stack-folded electrode assemblies, in which the same numeral refers to the same part.
Referring to FIGS. 1 to 3, electrode assemblies 10, 20, 30 comprises a plurality of unit cells 7a, 7b, 7c1, 7c2, each unit cell having a first separator 3a, 3b, 3c, and an anode 1a, 1b, 1c and a cathode 5a, 5b, 5c disposed on both sides of the first separator 3a, 3b, 3c. The cathode 5a, 5b, 5c has a structure that a cathode active material layer is formed on both surfaces of a cathode current collector, and the anode 1a, 1b, 1c has a structure that an anode active material layer is formed on both surfaces of an anode current collector. As shown in FIGS. 1 to 3, each unit cell has various structures, including a full-cell (7a, 7b) structure in which one cathode 5a, 5b and one anode 1a, 1b are disposed on both sides of the first separator 3a, 3b; and a bi-cell (7c1, 7c2) structure in which each first separator 3c is disposed on both surfaces of the cathode 5c or the anode 1c, and another cathode 5c or another anode 1c are each disposed on each first separator 3c, for example, a structure of cathode/separator/anode/separator/cathode or anode/separator/cathode/separator/anode.
In the electrode assemblies 10, 20, 30, each unit cell 7a, 7b, 7c1, 7c2 exists in the laminated form. Each of the unit cells 7a, 7b, 7c1, 7c2 which are neighboring and facing with each other is continuously surrounded with a second separator 9a, 9b, 9c alone, the separator 9a, 9b, 9c being disposed between the unit cells in various forms as shown in FIGS. 1 to 3, thereby performing the function as a separator.
Such a stack-folded electrode assembly is put into a battery case, to which an electrolyte solution is introduced, thereby preparing a battery. After the electrolyte solution is introduced, it takes time for the electrolyte solution to be sufficiently impregnated to the separator. Actually, it is difficult to obtain sufficient impregnation time owing to the problem of productivity. Accordingly, the electrolyte solution is not sufficiently impregnated to the separator, so the separator may less wet and the electrolyte solution not being impregnated may be leaked under severe conditions.
Further, gases generated from the decomposition of the electrolyte solution and the side reactions of the battery cause loosening phenomenon in the battery to deteriorate battery performances. If the gases generated from the side reactions fail to be discharged, it is difficult to inhibit the expansion of the battery. From this, the battery is deteriorated in its performances and is apt to be deformed by external impact, so the strength of the battery is lowered. Particularly, these problems are more likely to occur under high temperature conditions.